Effect of Chromium, Aluminum and Nickel on Microstructure and Reverse-S type Creep Rupture Strength of High Cr Ferritic Heat Resisting Steels

Abstract

Microstructure and creep strength at 650°C of two high-Cr ferritic heat resisting steels were investigated. 11Cr steel containing relatively high carbon, aluminum and nickel showed a significant drop in creep strength after 1000 h while 9Cr steel containing relatively low carbon, aluminum and nickel showed a recovery of creep strength after a slight drop. The 11Cr steel contained M₂₃(CB)₆, Z phase and relatively coarsened Laves phase as precipitates after 7000 h at 650°C, while the 9Cr steel contained fine MX and ultrafine M₆C in addition to M₂₃(CB)₆ and Laves phase. Z phase was not found and the growth of Laves phase was retarded in the 9Cr steel. It was concluded that the drop of creep strength in the 11Cr steel was attributed to the coarsened Laves and Z phases and the recovery of the strength in the 9Cr steel to fine MX and M₆C. Experimental results on the effects of steel compositions on the stability of precipitates in the steels were thermodynamically consistent with Thermo-Calc calculation results using an existing database. It was suggested that chromium and aluminum increased the kinetics of the formation and coarsening of the precipitates.